This invention relates to a skew compensation circuit for use in a magnetic tape controller interposed between a utilization device, such as an electronic computer, and a magnetic tape having a plurality of tracks.
In general, such a magnetic tape is driven or rotated along a rotation direction not only in a normal sense but also in a reverse sense. In addition, a plurality of readout signal sequences are simultaneously read out of the respective tracks of the magnetic tape by the use of a single magnetic head. In this event, the readout signal sequences should be read out of the magnetic tape with phases of the respective readout signal sequences matched with one another. However, it is practically difficult to match the phase of the respective readout signal sequences with one another due to expansion or shrinkage of the magnetic tape. As a result, phase differences or skews objectionably take place among the readout signal sequences read out of the magnetic tape of the above-mentioned type. Under the circumstances, the magnetic tape is controlled by a magnetic tape controller which usually comprises a skew compensation circuit for compensating for the skews among the readout signal sequences read out of the respective tracks.
A wide variety of signal formats are used to record or memorize on the magnetic tape a plurality of data signal sequences which are read out as the readout signal sequences with the signal formats kept unchanged. In this connection, description will be made only about the signal formats of the data signal sequences memorized on the magnetic tape, although specific description will not be made hereinafter. At any rate, each of the signal formats is similar to one another in that start and end identification codes are located to indicate a start position of the data signal sequences and an end position thereof, respectively. In addition, a plurality of resynchronization codes are interspersed between the start and the end identification codes so as to ensure synchronization of the magnetic tape during rotation of the magnetic tape. The start and the end identification codes and the resynchronization code are recorded on each track and are to be simultaneously reproduced from the respective tracks unless any skews or phase differences take place among the readout signal sequences.
A plurality of standard or first data portions are placed in the normal sense between the start identification code and a leading one of the resynchronization codes and between two adjacent ones of the resynchronization codes while an additional or second data portion is placed between a trailing one of the resynchronization codes and the end identification code. Each of the standard data portions has a standard or first length for arranging a predetermined number of data units each of which may be composed of nine bits. The predetermined number may be, for example, 70. The additional data portion has an additional or second length which is not longer than the first length and which is not invariable.
Under the circumstances, a conventional skew compensation circuit is operable in response to the readout signal sequences to compensate for the skews by detecting the start identification code, the end identification code, and the resynchronization codes and to successively produce the data units which are derived from the respective tracks and which are subjected to skew compensation. According to the conventional skew compensation circuit, it is possible to correctly determine positions of the resynchronization codes of the respective tracks by monitoring the first length of each of the standard data portions insofar as the magnetic tape is rotated in the normal sense.
Herein, it is assumed that the magnetic tape is rotated in the reverse sense so that readout operation is successively carried out from the end identification code towards the start identification code. In this case, the trailing one of the resynchronization codes is at first read out of each of the tracks after readout of the end identification codes. Inasmuch as the additional length of the second data portion is variable and is therefore not known in the readout signal sequences while the magnetic tape is rotated in the reverse sense, each trailing resynchronization code may variably be read out of each track. This shows that the trailing resynchronization codes might be variably detected from the respective tracks at different time instants. As a result, positions of the trailing resynchronization codes might be incorrectly determined by the conventional skew compensation circuit.